The embodiments described herein relate generally to aircraft engines and, more particularly, to aircraft engine starting and power generating systems and methods of control.
At least some known starter/generator systems for use with aircraft are used to both start an aircraft engine in a start mode, and to utilize the aircraft engine after it has started to provide electrical energy to power systems on the aircraft in a generate mode. For example, at least some known systems include a starter/generator with two stator exciter windings and a variable-voltage, variable-frequency power converter that is alternately connected to drive a dynamoelectric machine as a starting motor or to receive power from the dynamic electric machine during generator operation. In at least some systems, one winding is a multi-phase AC winding for use in start mode, and one winding is a DC winding for use in generate mode. However, such systems include winding-selective circuitry that is heavy and expensive. Moreover, exciter stator utilization in such systems is poor, and the conversion circuitry is large.
Moreover, at least some known systems have an inability of a synchronous exciter to generate sufficient excitation power for a main synchronous machine in a starter/generator at zero or low speeds. For example, at least some known systems supply a DC voltage to an exciter winding of a conventional exciter during a generate mode and an AC voltage to the same winding during a start mode. However, such systems require an AC supply voltage to the exciter to be higher than a maximum voltage that a DC bus or DC link voltage at the input of the inverter can be inverted to, which requires additional circuitry to boost the DC bus or DC link voltage. As a consequence, the higher AC voltage makes such systems more prone to corona effects.
Furthermore, at least some known systems use AC excitation for both start mode and generate mode. Such systems generally include a multi-phase AC exciter that is controlled to operate at a high, constant frequency during start mode in order to generate the AC voltage in an exciter armature that is located on a shaft through a rotating transformer effect. The output of the armature is connected to a rotating rectifier that is located on the same shaft, and that outputs a desired DC voltage to power the excitation winding of a main machine. During generate mode, the frequency of the supplied AC voltage switches from a high, constant frequency to a low, constant frequency. The AC voltage in the armature is generated again through the rotating transformer effect, which provides the excitation power to the main generator through the rotating rectifier. To use AC excitation in both modes, a higher exciter current is generally used in generate mode when compared with the current for power-generation-only applications. This results in the requirement of a large, heavy converter. The high current may be avoided, but requires the introduction of additional winding selective circuitry for use in the start mode.